I think the situation here is quite different from that of Twitter, at least for B2B chat apps. On the social network side, either you’re on Twitter or you’re not, but there is no real direct competitor. So when Twitter decides to shut off API access, as an app creator you lose access to all Twitter users at once and an individual user cannot move to another social network to keep benefiting from your app (since the contacts they used to interact with wouldn’t be there).
However, on the B2B chat platforms side, the situation is quite different because network effects are much weaker. Company A could be working on Slack and company B on HipChat. Let’s say an app builder creates a great integration with both Slack and HipChat. If company 1 starts depending on that app and Slack cuts their integration endpoints, the whole of company A can choose to migrate to HipChat, but this has little to no impact on company B. In other words, competition between chat platform providers should keep each other in check much more strongly.
If a chat app provider (be them Slack or HipChat) decided to cut API access, a typical company using integrations with 10–15 third-party services would have an easier time moving to HipChat than trying to build alternatives for all their use cases. And if Slack cuts API access to specific apps but not others, they’re much more likely to meet user backlash than if they cut it all at once. In short, I think the dynamics are quite different from what happened with Twitter.
Now obviously it doesn’t mean that Slack may not introduce new features that would supersede existing integrations or bots created by third-party providers, like the way Netscape could still work on Windows, but having IE installed by default took wind off its sails…